So many great lenses out there. I've got a lot of lenses. From 35mm to 280mm. Across copal and AF for the XF. But the two focal lengths I shoot with most on XF are the 55/80. And the 70mm on a tech cam. 10-15% of the time, I'll require a different focal length. For years I just shot with a 50mm on my Leica M2/M9M, then had to send the 50 in for service, so started using the 35mm, and kept using it even after the 50 came back. But lately, some of my best shots have been with the 80 on XF, and 70 on Tech cam.
If I had to pick one focal length, it's essentially the normal FOV. Now deciding on a platform... that's tough. I really like the seismic sensing on the XF. On the tech cam, I add a 2 sec timer. XF/80/WLF packs nice and small. But Tech cam only needs a single battery to run.